Chapter XIV — 251 — The Utricularia Trap 



the water pressure it must sustain when the trap is set, and (b) how the 

 water is prevented from leaking under the door edge. Recalhng the 

 structure of the threshold and especially that of the pavement, it is 

 necessary to point out that the latter along its middle reach is curved 

 in such a manner that it slopes somewhat, so as to face the interior of 

 the trap. At the bottom of the slope, where the inner zone begins, 

 there is an abrupt change in the direction of the slope so that a slight 

 depression is produced (29 — 4). Here the pavement is most closely 

 packed. The middle reach of the door edge is, as we have seen, 

 strengthened so as to make a firm edge, which rests against the pave- 

 ment just in or beyond the depression, its outer selvage surface resting 

 more or less against the pavement, according to the amount of strain 

 produced by water pressure. When the water pressure is greatest, 

 that is, when the trap is fully set, the position of the door edge is 

 more nearly normal to the pavement than when the trap has just been 

 released. This can be inferred from the measurements of photographs 

 of traps before and just after "firing" (Lloyd 1932&) {29 — 11). It is 

 indicated also by the position of the bristles, which are more erect 

 when the trap is in the set condition. It may be added here that the 

 whole shape of the trap is altered a little by the change in postures of 

 the side walls. Since the ends of the door edge coincide with the inner 

 angles of the threshold, it follows that its lateral reaches cannot follow 

 the pavement parallel to its midline. It is only its middle reach (the 

 middle piece) which impinges edgewise on the pavement. The lateral 

 reaches merely lie with the outer surface of the selvage flat against the 

 pavement, thus forming a re-entrant sHt through which the water must 

 leak under pressure unless this contingency were provided against, 

 which is the case. The cuticular membrane, the velum, attached to 

 the outer zone of the pavement, is slung completely across from end 

 to end of the threshold (25 — 5-8). When the door swings outwardly 

 after springing the trap, it pushes against the velum which folds 

 against the door {2j — 4), covering the re-entrant shts on the sides and 

 blocking the door edge in the middle. When the door is in position, 

 the velum reaches in front up to the short spherical hairs which stand 

 in a curved row just below the level of the tripping hairs (24 — i ; 

 2j — 3-5)- Experimental proof that the velum thus blocks the 

 entrance by its valvular action, consisted in cutting the side reach. 

 This was accomplished with a very small knife several times and the 

 parts carefully examined afterwards for assurance that no other damage 

 had been done (Lloyd 1932&). In no case after the velum was cut 

 did the trap reset itself. In Czaja's experiment in which he thrust a 

 hair beneath the door edge, this not only held up the door edge but 

 depressed the velum also, but this escaped his attention. 



As CzAjA found, the distortion of the door edge (and that of the 

 velum at the same time) when it rests on a hair, prevents the traps 

 from working. This does not seem to be the case if the entrance is 

 filled with the soft body of a large capture, sufficient to plug it {20 — 

 11). Matheson (1930) states that such prey may eventually be 

 ingested, indicating that the trap, plugged by the animal's body, still 

 evacuates its water. In the meantime the prey may be softened and 

 respond to the sucking action when re-established, and thus eventually 



